The invention relates to an apparatus for producing a gas mixture by the saturation method, wherein a stock vessel is provided for storing a liquid to be mixed with a carrier gas and a dip tube is provided for introducing the carrier gas into the liquid.
Such devices are used for producing a continuous volumetric flow of a defined mixture of a carrier gas and an additive. The saturation method according to current practice (VDI guideline 3490, sheet 13) involves passing the carrier gas through the liquid (condensed phase of the additive) maintained at a defined temperature and thus saturating it. The volume content of the additive is determined by the partial pressure of the additive, relative to the total pressure of the gas mixture at the particular temperature.
These prior art devices are used as sources of substances for special processes in semiconductor manufacture, such as, for example, for chemical vapor deposition processes, doping processes and diffusion processes, but additionally, also for deposition processes for the production of optical fibers.
The particular difficulties of these methods consist above all in maintaining an accurately constant concentration of the additive in the gas mixture. In the production of optical gradient fibers, it is also necessary to run with a precisely predetermined concentration profile.
It is a common feature of all the devices of the generic type thus far described that the saturation of the carrier gas take place in the stock vessel containing the liquid. The carrier gas is introduced through a dip tube into the liquid and bubbles upwards while being saturated with the additive from the liquid phase. The resulting degree of saturation depends on the path length covered by the gas bubbles through the liquid and, therefore, changes when the level in the stock vessel decreases.
In order to eliminate temperature effects, the entire stock vessel must be maintained at the desired temperature by means of a temperature control system. Therefore, after an exchange of the stock vessel, which is also called feed vessel, the device is ready for operation only after a prolonged period, when the entire liquid stock has reached the predetermined temperature. The desired change in the liquid temperature can therefore usually be set only after a wait of several hours.
These known devices for producing a gas mixture by the saturation process are also called "bubblers" because of the gas bubbles rising from the dip tube or fritted tube; the volumetric flow is regulated by controlling the mass flow the carrier gas.